Assay test kits are currently available for testing a wide variety of medical and environmental conditions or compounds, such as a hormone, a metabolite, a toxin, or a pathogen-derived antigen. FIG. 1 shows a typical lateral flow test strip 10 that includes a sample receiving zone 12, a labeling zone 14, a detection zone 15, and an absorbent zone 20 on a common substrate 22. These zones 12-20 typically are made of a material (e.g., chemically-treated nitrocellulose) that allows fluid to flow from the sample receiving zone 12 to the absorbent zone 22 by capillary action. The detection zone 15 includes a test region 16 for detecting the presence of a target analyte in a fluid sample and a control region 18 for indicating the completion of an assay test.
FIGS. 2A and 2B show an assay performed by an exemplary implementation of the test strip 10. A fluid sample 24 (e.g., blood, urine, or saliva) is applied to the sample receiving zone 12. In the example shown in FIGS. 2A and 2B, the fluid sample 24 includes a target analyte 26 (i.e., a molecule or compound that can be assayed by the test strip 10). Capillary action draws the liquid sample 24 downstream into the labeling zone 14, which contains a substance 28 for indirect labeling of the target analyte 26. In the illustrated example, the labeling substance 28 consists of an immunoglobulin 30 with a detectable particle 32 (e.g., a reflective colloidal gold or silver particle). The immunoglobulin 30 specifically binds the target analyte 26 to form a labeled target analyte complex. In some other implementations, the labeling substance 28 is a non-immunoglobulin labeled compound that specifically binds the target analyte 26 to form a labeled target analyte complex.
The labeled target analyte complexes, along with excess quantities of the labeling substance, are carried along the lateral flow path into the test region 16, which contains immobilized compounds 34 that are capable of specifically binding the target analyte 26. In the illustrated example, the immobilized compounds 34 are immunoglobulins that specifically bind the labeled target analyte complexes and thereby retain the labeled target analyte complexes in the test region 16. The presence of the labeled analyte in the sample typically is evidenced by a visually detectable coloring of the test region 16 that appears as a result of the accumulation of the labeling substance in the test region 16.
The control region 18 typically is designed to indicate that an assay has been performed to completion. Compounds 35 in the control region 18 bind and retain the labeling substance 28. The labeling substance 28 typically becomes visible in the control region 18 after a sufficient quantity of the labeling substance 28 has accumulated. When the target analyte 26 is not present in the sample, the test region 16 will not be colored, whereas the control region 18 will be colored to indicate that assay has been performed. The absorbent zone 20 captures excess quantities of the fluid sample 24.
In the non-competitive-type of lateral flow assay test strip designs shown in FIGS. 2A and 2B, an increase in the concentration of the analyte in the sample results in an increase in the concentration of labels in the test region. Conversely, in competitive-type of lateral flow assay test strip designs, an increase in the concentration of the analyte in the fluid sample results in a decrease in the concentration of labels in the test region.
Lateral flow assay test strips may include multiple test regions that include different respective immobilized compounds that specifically bind different respective target analytes. Pall Corporation has published an article entitled “Immunochromatographic, Lateral Flow or Strip Tests Development Ideas” that proposes that such test strips might be used to create a visually-detectable drugs-of-abuse test and visually-detectable panels for which multiple analytes can be tested, such as immune diseases, allergies, or even Multiple Chemical Sensitivity Disorder.
Although visual inspection of lateral flow assay devices of the type described above are able to provide qualitative assay results, such a method of reading these types of devices is unable to provide quantitative assay measurements and therefore is prone to interpretation errors. Automated and semi-automated lateral flow assay readers have been developed in an effort to overcome this deficiency. These readers typically include a light source and one or more sensors that detect the intensity of light from the labels that are immobilized in the capture regions (e.g., the test and control regions) of the test strip. Many of these readers determine the concentrations of the target analytes in the sample by measuring the intensities of the light signals from each test region and subsequently quantifying the measured light signals with respect to a baseline signal or a calibration signal that is obtained from one or more calibration regions on the test strip. Such readers require significant computing and processing resources to determine single-factor (i.e., concentrations of individual target analytes) assays of test samples.
What are needed are systems and methods of performing rapid, low-cost multifactor assays, such as multi-analyte screening tests, on test strips that have multiple capture reagents.